Worm gear mechanisms include drive-side worms and torque transmission worm wheels meshing with the worms for transmitting torque from the worms to a load side. Various types of worm gear mechanisms have been developed for eliminating backlashes (as disclosed in, for example, JP-A-2001-355700 and JP-A-2002-37100).
Discussion will be made as to a worm gear mechanism disclosed in JP-A-2001-355700 with reference to FIG. 13A through FIG. 13C hereof. Discussion will be made as to a worm gear mechanism disclosed in JP-A-2002-37100 with reference to FIG. 14A and FIG. 14B hereof.
Referring to FIG. 13A, the worm gear mechanism designated at reference numeral 200 is shown as being connected to an electric motor 201. FIG. 13B shows in cross-section the worm gear mechanism 200. FIG. 13C shows a meshing engagement provided in the worm gear mechanism 200.
As shown in FIG. 13A, the worm gear mechanism 200 includes a drive-side worm 202 connected to the electric motor 201, and a driven side worm wheel 204 meshing with the worm 202 and coupled to an output shaft 203. A worm shaft 205 is connected to an output shaft of the electric motor 201.
The worm wheel 204 includes a hub 206 coupled to the output shaft 203, first and second gears 207, 208 disposed about an outer periphery of the hub 206, and an elastic member 209 elastically interconnecting the first and second gears 207, 208 and an outer peripheral surface of the hub 208, as shown in FIG. 13A through FIG. 13C.
The worm wheel 204 meshing with the worm 202 is divided into the two gears (the first and second gears 207, 208) spaced from each other in a direction in which a rotational axis of the worm wheel 204 extends. The worm wheel 204 is urged by the elastic member 209 in a direction of rotation, with the gears 207, 208 disposed out of phase with each other.
In the worm gear mechanism 200, a tooth 207a of the first gear 207 and a tooth 208a of the second gear 208 which are disposed on opposite sides of a tooth 202a of the worm 202 cooperate with each other to sandwich the tooth 202a therebetween for eliminating a backlash.
As for the worm gear mechanism 200, any tooth 207a or 208a of each gear has a contact surface area, which is below half the entire surface area of the tooth 207a or 208a, for contact with the tooth 202a of the worm 202 because the worm gear mechanism 200 has the halved structure, that is, the two gears 207, 208 spaced from each other in the direction of the extension of the rotational axis of the worm wheel 204. Forward rotation of the worm 202 transmits a torque to the tooth 207a of the first gear 207. Reverse rotation of the worm 202 transmits a torque to the tooth 208a of the second gear 208. When contacting the worm 202, the worm wheel 204 undergoes a maximum contact pressure at a dividing portion (positioned centrally in a left-and-right direction of FIG. 13B) where the first gear 207 is separated from the second gear 208. Therefore, there is left a room for improvement in durability, especially, in abrasion resistance of the worm gear mechanism 200.
The worm gear mechanism disclosed in JP-A-2002-37100 is schematically explained with reference to FIG. 14A and FIG. 14B. FIG. 14A shows the worm gear mechanism designated at 300 and connected to an electric motor 301. FIG. 14B shows in cross-section the worm gear mechanism 300.
The worm gear mechanism 300 shown in FIG. 14A includes a drive-side worm 302 connected to the electric motor 301, and a driven side worm wheel 304 meshing with the worm 302 and coupled to an output shaft 303. The worm 302 is connected to a motor shaft 305 by means of a worm shaft.
As shown in FIG. 14B, the worm wheel 304 includes a tooth 311 having a portion (a shaded portion) meshing with a tooth 302a of the worm 302. Such a portion is called a meshing region 312.
The tooth 311 of the worm wheel 304 has an annular retaining groove 313 formed on one side of the meshing region 312 in a face width direction of the tooth 311. Within the retaining groove 313, there is mounted an O-ring 321 made of rubber. The rubber-made O-ring 321 is flexed slightly by contacting a tooth crest 302b of the worm 302 and applies a pre-load to the meshing portion by an elastic recovering force of the O-ring 321 for eliminating a backlash.
In the worm gear mechanism 300 shown in FIG. 14B, the tooth 311 does not have any retaining groove other than the retaining groove 313 formed at the one side of the meshing region 312 in the face width direction (the left-and-right direction of FIG. 14B) of the tooth 311. Thus, the one side of the meshing region 312 in the face width direction differs from the opposite side of the meshing region 312 in flexural rigidity in a tooth thickness direction. Accordingly, the one side of the meshing region 312 in the face width direction differs from the opposite side of the meshing region 312 in contact pressure applied from the worm 302, and hence there is left a room for improvement in durability of the worm gear mechanism 300.
Additionally, a friction force is produced by a contact between the tooth crest 302b of the rotational worm 302 and the rubber-made O-ring 321. The rubber-made O-ring 321 has its relatively large radius which is a distance between a contact surface of the O-ring 321 and a center on which the worm wheel 304 rotates, due to which a large friction torque is produced. For an increased torque transmission efficiency of the worm gear mechanism 300, it is preferable to reduce such a large friction torque.
Moreover, with a frequent contact between the tooth crest 302b and the rubber-made O-ring 321 being taken into consideration, there is a room for improvement in durability of the O-ring 321.
There is a demand for a worm gear mechanism having an improved durability and providing a satisfactory meshing engagement between a worm and a worm wheel in addition to reducing or suppressing a strike sound between a tooth of the worm and a tooth of the worm wheel.